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Technical Abstract: Are Historic Disturbance Regimes Appropriate in Sagebrush Plant Communities under Modern Conditions?
Historic disturbance regimes are often considered a critical element in maintaining native plant communities. However, the response of plant communities to disturbance may be altered as a consequence of invasive plants and/or altered environmental conditions. We evaluated the response of Wyoming big sagebrush plant communities to their historic disturbance regime compared to other disturbance regimes. The historic disturbance regime of these plant communities was periodic fires with minimal grazing by large herbivores. Treatments were: 1) ungrazed (livestock grazing excluded since 1936) unburned, 2) grazed unburned, 3) ungrazed burned (burned in 1993), and 4) grazed burned. The ungrazed burned treatment emulated the historic disturbance regime. Vegetation cover, density, and biomass production were measured the 12th, 13th, and 14th year post-burning. Prior to burning, cheatgrass, an exotic annual grass, presence was minimal (< 0.5% cover) and vegetation characteristics were similar between the grazed and ungrazed treatments. However, litter accumulation was almost 2-fold greater in the ungrazed than grazed treatments. Long-term grazing exclusion followed by burning resulted in a substantial cheatgrass invasion and a decrease in perennial herbaceous vegetation, but burning the grazed areas did not produce an invasion. The accumulation of litter in the ungrazed treatments may have resulted in greater fire-induced mortality of perennial vegetation in the ungrazed compared to the grazed treatments. Our results suggest that modern deviations from historic conditions can alter ecosystems response to disturbances, thus restoring the historic disturbance regime may not be an appropriate strategy for all ecosystems.